Hydraulic power steering device

ABSTRACT

A hydraulic power steering apparatus  1  which is arranged so that a damper valve  10  is disposed in hydraulic oil circuits  8  and  9  for connecting an oil passage selector valve  11  in a gear box with right and left oil chambers of a power cylinder  6 . The damper valve  10  comprises a spool valve. And a control oil chamber  21  of the spool valve  10  is connected with a hydraulic oil feed passage  18  for connecting a hydraulic pump  13  which is driven by an internal combustion engine so as to discharge hydraulic oil, with the oil passage selector valve  11 . Its back pressure chamber  17  is connected with a hydraulic oil circulating passage  19  for connecting the oil passage selector valve  11  with a reservoir tank  12 . Orifices  27  and  28  are provided in connecting oil passages  22  and  20 , such that when discharge oil pressure of the hydraulic pump  13  rises, opening of the spool valve  10  increase.

This is a 371 of International application PCT/JP99/03753, with aninternational filing date of Jul. 12, 1999.

TECHNICAL FIELD

The present invention relates to a vehicle-use hydraulic power steeringapparatus useable in a vehicle, the apparatus having a damper valve. Inparticular, the invention relates to a hydraulic power steeringapparatus in which the dampering effect is changeable according to thenumber of revolutions of an internal combustion engine so that steeringfollow-up performance in a low speed range of the vehicle is improvedand stability of steering in a high speed range is improved.

BACKGROUND ART

Conventionally, Japanese Utility Model Application Publication No.2-49109 discloses a hydraulic power steering apparatus used in a vehiclein which a damper valve is disposed in a hydraulic oil circuit forconnecting an oil passage selector valve in a gear box with right andleft oil chambers of a power cylinder.

In this apparatus, as shown in FIG. 5, hydraulic oil circuits 08 and 09,which connect an oil passage selector valve 011 housed in a gear box ofa hydraulic power steering apparatus 01 with right and left oil chambersof a power cylinder 06, is provided with check valves 010 a, 010 a whichallow oil only to flow from the oil passage selector valve 011 to theright and left oil chambers, and throttle check valves 010 b, 010 bwhich have an initial load and allow oil only from the oil chambers tothe oil passage selector valve 011. The check valves 010 a, 010 a andthe throttle check valves 010 b, 010 b comprise a damper valve.

A hydraulic pump 013 which is a hydraulic source for supplying hydraulicoil to the power cylinder 06 includes a flow control valve 023, a reliefvalve 024 and a fixed orifice 025, and their cooperation allows thehydraulic oil to be discharged by a discharge amount in compliance witha characteristic chart shown in FIG. 4 according to the number ofrevolutions.

In other words, the hydraulic oil which gradually increases and reachesa substantially constantly high level is discharged in a low revolvingrange where the number of revolutions of the hydraulic pump 013 reachesa predetermined low revolving speed N, and when it exceeds the number ofrevolutions N, the hydraulic oil whose amount gradually decrease andreaches a substantially constantly low level is discharged. Thehydraulic pump 013 is connected to a crank shaft of an internalcombustion engine, not shown, via a transmission mechanism, and itrotates at the number of revolutions which is the same as the number ofrevolutions of the internal combustion engine.

Since the damper valve in the above-mentioned conventional hydraulicpower steering apparatus 01 does not have a portion where friction isgenerated, returning performance of a steering handle is excellent.Moreover, since each of the throttle check valves 010 b, 010 b has theinitial load, even if a rack shaft 04 which serves also as a piston rodof the power cylinder 06 tries to slightly oscillate due to unbalance oftires or the like, the flow of the hydraulic oil from the right and leftoil chambers to the oil passage selector valve 011 is restricted, andthus the rack shaft 04 hardly moves. For this reason, slight oscillationof the steering handle, namely, a shimmy phenomenon, can be prevented.In this regard, reference numeral 012 is a reservoir tank.

However, since the conventional damper valve has a constant dampereffect regardless of the number of revolutions of the internalcombustion engine (the number of revolution of the hydraulic pump) andthe speed of a vehicle, the damper effect is not required so much at thetime of low-speed revolution of the internal combustion engine and in alow-speed operation of the vehicle. In fact the damper effect isrequired in the occasion opposite to the above one. However, in the pastthe damper effect could not be adjusted according to the degree ofnecessity. For this reason, there is a need to improve steeringfollow-up performance of the steering handle particularly, at the lowspeed of the vehicle.

DISCLOSURE OF THE INVENTION

The present invention is disclosed in order to solve the above problem.It therefore is an object of the present invention to provide ahydraulic power steering apparatus in which the damper effect of adamper valve is changeable according to the number of revolutions of aninternal combustion engine so that steering follow-up performance of asteering handle, particularly in a low speed range of a vehicle can beimproved, and stability of steering in a high speed range of the vehiclecan be improved, and a shimmy phenomenon and the kick-back phenomenoncan be prevented simultaneously.

The present invention relates to a hydraulic power steering apparatuswhich solves the above problem. The invention provides a hydraulic powersteering apparatus, which is constituted so that a damper valve isdisposed in hydraulic oil circuits for connecting an oil passageselector valve in a gear box with right and left oil chambers of a powercylinder, characterized in that: the damper valve is composed of a spoolvalve. A control oil chamber of the spool valve is connected with ahydraulic oil feed passage for connecting a hydraulic pump which isdriven by an internal combustion engine so as to discharge hydraulic oilwith the oil passage selector valve. Its back pressure chamber isconnected with a hydraulic oil circulating passage for connecting theoil passage selector valve with a reservoir tank. Orifices are providedrespectively in the connecting oil passages, and when a discharged oilpressure of the hydraulic pump rises, the opening of the spool valveincreases.

Since the invention is constituted as mentioned above, the spool valve,which composes the damper valve disposed in the hydraulic oil circuitsfor connecting the oil passage selector valve with the right and leftoil chambers of the power cylinder, is constituted so that its controloil chamber is connected with the hydraulic oil feed passage forconnecting the hydraulic pump with the oil passage selector valve. Itsback pressure chamber is connected with the hydraulic oil circulatingpassage for connecting the oil passage selector valve with the reservoirtank. When the spool valve is actuated in compliance with a dischargeoil pressure of the hydraulic pump, which is driven by the internalcombustion engine so as to discharge hydraulic oil and the discharge oilpressure rises, the opening of the spool valve increases.

As a result, when the number of revolutions of the internal combustionengine is low and thus the number of revolutions of the hydraulic pumpis low, and a discharge amount of the hydraulic oil increases based on acharacteristic chart of the power steering, when a steering handle issteered, a discharge pressure of the hydraulic oil also rises. As aresult, opening of the damper valve increases, and thus the hydraulicoil circuits are opened to a greater extent. As a result, the hydraulicoil easily flows through the circuits, the damper effect of the dampervalve is weakened, and steering follow-up performance of the steeringhandle in a low-speed range of a vehicle is improved so that an operatorfeels lightness of steering.

In addition, in the case where the number of revolutions of the internalcombustion engine is high, and thus the number of revolutions of thehydraulic pump is high and a discharge amount of the hydraulic oilbecomes small based on the characteristic chart of the power steering,even when the steering handle is steered, the discharge pressure of thehydraulic oil does not rise so much. As a result, the opening of thedamper valve becomes small so that the hydraulic oil circuits are openedto a lesser extent. As a result, the hydraulic oil flows withdifficultly through the circuits, and the damper effect of the dampervalve is manifested. Then, the steering follow-up performance of thesteering handle in a high-speed range of the vehicle is lowered, andinstability of the steering wheel in a neutral position is reduced, sothat the operator feels stability of the steering.

Further, in this case, since the damper valve displays the essentialdamper effect, slight oscillation (shimmy phenomenon), which istransmitted from the wheels to the steering handle via a piston rod(rack shaft) of the power cylinder while the vehicle is running, issuppressed. Moreover, a so-called kick-back phenomenon such that thesteering handle rotates in a reverse manner, which occurs when thewheels run onto a rock or the like, is also suppressed. As a result,comfortableness of the steering is improved.

In addition, since orifices are disposed respectively in a connectingoil passage for connecting the control oil chamber of the damper valve(spool valve) with the hydraulic oil feed passage and in a connectingoil passage for connecting the back pressure chamber with the hydraulicoil circulating passage, the movement of a spool of the damper valve isslowed down. As a result, when the steering handle is returned to itsneutral position, particularly in a low-speed range of a vehicle, thesteering handle can be returned before the damper effect is generated sothat the operator feels the steering being more light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a whole structural diagram of a hydraulic power steeringapparatus according to one embodiment of the present invention.

FIG. 2 is a schematic structural diagram, with portions broken away,showing the hydraulic power steering apparatus of FIG. 1.

FIG. 3 is a partially enlarged diagram of FIG. 2.

FIG. 4 is a characteristic chart of a hydraulic pump in the hydraulicpower steering apparatus of FIG. 1.

FIG. 5 is a diagram showing a conventional example.

BEST MODE FOR CARRYING OUT THE INVENTION

There will be described below one embodiment of the present inventionshown in FIGS. 1 through 4.

FIG. 1 is a whole structural diagram of a hydraulic power steeringapparatus according to the present embodiment, FIG. 2 is a schematicstructural diagram, with parts broken away, of the hydraulic powersteering apparatus of FIG. 1, FIG. 3 is a partially enlarged diagram ofFIG. 2, and FIG. 4 is a characteristic chart of a hydraulic pump in thehydraulic power steering apparatus of FIG. 1.

In FIG. 1, the hydraulic power steering apparatus 1 according to thepresent embodiment is a rack and pinion type power steering apparatus.In this apparatus, an input shaft 2 which is connected with a steeringhandle of a vehicle, not shown, is connected with an output shaft(pinion shaft), not shown, via a torsion bar in a selector valve housingportion 3 a of a gear box 3. And a rack shaft 4 having a rack engagedwith a pinion formed on the output shaft is housed in a rack shaft powercylinder housing portion 3 b of the gear box 3. so as to slide right andleft as shown in FIG. 1.

Both ends of the rack shaft 4 are connected respectively with a pair ofright and left tie rods 5, 5. A middle portion of the rack shaft 4 isconnected with a piston 7 of a power cylinder 6 housed in the rack shaftpower cylinder housing portion 3 b. Therefore, the rack shaft 4 servesalso as a piston rod of the piston 7.

An oil passage selector valve 11 (rotary valve: see FIG. 2) is housed inthe selector valve housing portion 3 a of the gear box 3. The oilpassage selector valve 11 is opened in compliance with a torsion amountof the torsion bar, namely, a relative difference in a rotating amountbetween the input shaft 2 and the output shaft. This feeds hydraulicoil, which is pressure-fed from the hydraulic pump 13 as a hydraulic oilpressure source via a hydraulic oil feed passage 18, to one of the rightand left oil chambers of the power cylinder 6 via a left cylinder oilpassage 8 or a right cylinder oil passage 9, and circulates thehydraulic oil, which is returned from the other one of the right andleft oil chambers of the power cylinder 6 via the right cylinder oilpassage 9 or the left cylinder oil passage 8, to a reservoir tank 12 viaa hydraulic oil circulating passage 19.

The hydraulic pump 13 is connected with a crank shaft, not shown, of aninternal combustion engine via a transmission mechanism, and it rotatesat the number of revolutions equal to the number of revolution of theinternal combustion engine. As schematically shown in FIG. 2, thehydraulic pump 13 has a flow control valve 23, a relief valve 24, and afixed orifice 25. Their cooperation discharges the hydraulic oil of adischarge amount in compliance with the characteristic chart shown inFIG. 4 according to the number of revolutions. The characteristic chartis the same as a characteristic chart of the conventional hydraulic pump013.

Therefore, when the pinion of the output shaft is engaged with the rack,and the rack shaft 4 moves right and left, the tie rods 5, 5 integrallymove right and left in compliance with the right and left movement ofthe rack shaft 4 so that right and left wheels are steered. Thehydraulic oil, which is discharged from the hydraulic pump 13 incompliance with the number of revolutions of the internal combustionengine, and is selectively controlled by the oil passage selector valve11, is fed to one of the right and left oil chambers of the powercylinder 6. As a result, the piston 7, the rack shaft 4 and the tie rods5, 5 integrally move right and left so that superposable steering of theright and left wheels are executed based on steering auxiliary power.

Here, as shown in FIG. 1, a damper valve 10 is mounted to a casing ofthe selector valve housing portion 3 a of the gear box 3.

As detailed in FIGS. 2 and 3, the damper valve 10 comprises a spoolvalve, which is disposed across the right and left cylinder oil passages(hydraulic oil circuits) 8 and 9 which connect the oil passage selectorvalve 11 with the right and left oil chambers of the power cylinder 6.The damper valve 10 can open and/or close these oil passagessimultaneously so as to be capable of adjusting the damping effect.

Therefore, when the connecting oil passages 8 and 9 from the dampervalve 10 to the power cylinder 6 are throttled so that the dampingeffect is enhanced, even when oscillation is transmitted from the wheelsto the power cylinder 6 while the vehicle is running, the right and leftmovements of the piston 7 and the rack shaft 4 are suppressed. For thisreason, the transmission of the oscillation to a steering handle, notshown, via the rack shaft 4 can be suppressed. As a result, occurrenceof the shimmy phenomenon, such that the steering handle slightlyoscillates, can be prevented.

In addition, occurrence of the so-called kick-back phenomenon, such thatthe steering handle rotates in reverse manner when the wheels run onto arock or the like, can be also prevented for a similar reason. As aresult, steering comfort is improved.

The above-mentioned damper effect of the damper cylinder 10 ischangeable in compliance with the number of revolutions of the internalcombustion engine as will be mentioned later.

There will be described below a detailed structure and function of thedamper valve 10 and effects of the present embodiment.

As shown in FIG. 3 in detail, the damper valve 10 is housed in a spoolhousing hole 16 of a valve casing (sleeve) 26 such that a force isapplied to a spool 14 in one direction (left direction in FIG. 3) by aspring 15, and circular U-shaped grooves 14 b and 14 c are formed inboth sides of a land portion 14 a at a center of the spool 14.

The connecting right and left cylinder oil passages 8 and 9 from thedamper valve 10 to the power cylinder 6 contact with separate portportions 8 a and 9 a which are opened to the spool housing hole 16, andthe circular U-shaped grooves 14 b and 14 c adjust the opening of theconnecting right and left cylinder oil passages 8 and 9 according to adegree of superposing of the connecting oil passages 8 and 9 with theport portions 8 a and 9 a.

A control oil chamber 21, to which hydraulic oil pressure for moving thespool 14 of the damper valve 10 is applied, is formed on a left end ofthe spool housing hole 16 in FIG. 3. The control oil chamber 21 isconnected with a discharge side of the hydraulic pump 13 via an oilpassage 22 and the feed passage 18. A back pressure chamber 17 on aright end of the spool housing hole 16 in which a spring 15 is housed isconnected with the reservoir tank 12 via an oil passage 20 and thecirculating passage 19. Fixed orifices 27 and 28 are disposedrespectively in midway portions of the oil passages 22 and 20 close tothe valve casing 26.

Therefore, when the number of revolutions of the internal combustionengine is low, and the number of revolutions of the hydraulic pump 13 isalso lower than the characteristic chart N of FIG. 4, and a dischargeamount of the hydraulic oil is comparatively large, when the steeringhandle is steered, a throttling action occurs at the oil passageselector valve 11 causing discharge pressure of the hydraulic oil torise. As a result, high pressure of the hydraulic oil is applied to thecontrol oil chamber 21 of the damper valve 10, moving the spool 14 rightin FIG. 2 against the force of the spring 15.

As a result, since the circular U-shaped grooves 14 b and 14 c of thespool 14 move in a direction where the connecting right and leftcylinder oil passages 8 and 9 are superposed on the port portions 8 aand 9 a by a greater amount, the connecting right and left cylinder oilpassages 8 and 9 are opened more (opening of the damper valve 10 isrelatively large. Thus, the hydraulic oil easily flows through the oilpassages and the damper effect of the damper valve 10 is weakened.

When the damper effect of the damper valve 10 is weakened in such amanner, the right and left movement of the rack shaft 4 becomes smooth,and thus the steering of the steering handle becomes light. As a result,the steering is more easily executed so that the steering follow-upperformance is improved. Furthermore, since follow-up performance of thereturn of the steering handle is also improved, an operator feels thesteering being light. In such a manner, normal power assist is executed.

Moreover, when the steering handle is returned, the movement of thespool 14 of the damper valve 10 is slowed down by the actions of theorifices 27 and 28. As a result, before the damper valve 10 starts todisplay the damper effect, the steering handle can be quickly returnedto its neutral position, and thus the operator feels the steering beinglighter.

In this case, the slight oscillation, which is transmitted from thewheels, not shown, to the power cylinder 6 while the vehicle is running,is slightly transmitted to the steering handle via the right and leftmovement of the rack shaft 4, and thus the shimmy phenomenon occurs.However, since the internal combustion engine rotates at a low speed,there arises no problem.

Next, in the case where the number of revolutions of the internalcombustion engine is high and the number of revolutions of the hydraulicpump 13 is also higher than the characteristic chart N of FIG. 4, and adischarge amount of the hydraulic oil is comparatively small, even ifthe steering handle is steered and the throttling action is taken at theoil passage selector valve 11, the discharge pressure of the hydraulicoil does not rise so much. As a result, a low pressure of the hydraulicoil is applied to the control oil chamber 21 of the damper valve 10, andthe spool 14 is moved left in FIG. 2 by the force of the spring 15.

Then, since the circular U-shaped grooves 14 b and 14 c of the spool 14move in a direction where the connecting right and left cylinder oilpassages 8 and 9 separate from the port portions 8 a and 9 a, theconnecting right and left cylinder oil passages 8 and 9 are throttled(opening of the damper valve 10 is relatively small), and thus thehydraulic oil flows at a relatively low rate through these oil passagesso that the damper effect of the damper valve 10 is enhanced.

When the damper effect of the damper valve 10 is enhanced in such amanner, the rack shaft 4 is not moved smoothly right and left. As aresult, the steering of the steering handle becomes heavy, and thesteering is hardly executed so that the steering follow-up performanceis lowered. As a result, instability of the steering handle in theneutral position at a high speed of the vehicle is reduced so that theoperator feels the steering being stable.

In this case, since the damper valve 10 displays the essential dampereffect, generation of the slight oscillation (shimmy phenomenon), whichis transmitted from the wheels, not shown, to the steering handle viathe rack shaft 4 while the vehicle is running, is suppressed. Moreover,the kick-back phenomenon such that the steering handle rotates in areverse manner, which occurs when the wheels run onto a rock or thelike, is also suppressed for similar reasons. As a result, steeringcomfort is improved.

In the present embodiment, the damper valve 10 is mounted to the casingof the selector valve housing portion 3 a of the gear box 3, but thepresent invention is not limited to this, and thus it may be mounted toany place possible from a layout viewpoint which is in the hydraulic oilcircuits 8 and 9 for connecting the oil passage selector valve 11 in theselector valve housing portion 3 a with the right and left oil chambersof the power cylinder 6.

INDUSTRIAL APPLICABILITY

According to the present invention, in the hydraulic power steeringapparatus having the damper valve, the damper effect is changeable incompliance with the number of revolutions of the internal combustionengine so that the steering follow-up performance in a low-speed rangeof the vehicle is improved and the stability of the steering in ahigh-speed range is improved, and the occurrence of the shimmyphenomenon and the kick-back phenomenon can be prevented.

What is claimed is:
 1. A hydraulic power steering apparatus comprising:a hydraulic pump, a reservoir tank, an oil passage selector valve, whichselects between a right and or a left hydraulic flow path a powercylinder having right and left oil chambers, and a damper valve composedof a spool valve having a single spool valve body, a control oilchamber, a back pressure chamber with a bias spring, and right and leftoil passages; said reservoir tank hydraulically interconnected betweensaid oil passage selector valve, said spool valve back pressure chamberand an inlet of said hydraulic pump; said hydraulic pump having anoutlet hydraulically interconnected with said damper valve control oilchamber and said oil passage selector valve; said damper valve right andleft oil passages hydraulically interconnected between said oil passageselector valve right and left hydraulic flow paths and said powercylinder right and left oil chambers by left and right connecting oilpassages; said spool valve operates upon said connecting right and leftoil passages, opening or closing said connecting right and left oilpassages at the same time; whereby when a discharge oil pressure of saidhydraulic pump decreases, a pressure in said control oil chamberdecreases allowing said bias spring to move said spool valve to aposition restricting said connecting right and left oil passages.
 2. Ahydraulic power steering apparatus comprising: a hydraulic pump which isdriven by an internal combustion engine, a reservoir tank, an oilpassage selector valve in a gear box, which selects between a right andor a left hydraulic flow path, a power cylinder with right and left oilchambers, and a damper valve composed of a single spool valve having abody, a control oil chamber, a back pressure chamber with a bias spring,a right oil passage and a left oil passage; said reservoir tankhydraulically interconnected between an outlet of said oil passageselector valve, said spool valve back pressure chamber and an inlet ofsaid hydraulic pump; said hydraulic pump having an outlet hydraulicallyinterconnected with said damper valve control oil chamber and an inletof said oil passage selector valve; said damper valve right and left oilpassages hydraulically interconnected between said oil passage selectorvalve right and left hydraulic flow paths and said power cylinder rightand left oil chambers by left and right connecting oil passages; saidspool valve operates upon said connecting right and left oil passages,opening or closing said connecting right and left oil passages at thesame time; whereby when a discharge oil pressure of said hydraulic pumpdecreases due to an increase in a revolution rate of said internalcombustion engine, a pressure in said control oil chamber decreasesallowing said bias spring to move said spool valve body to a positionrestricting said connecting right and left oil passages therebyincreasing a damper effect upon said power cylinder.